Light source using large area LEDs

ABSTRACT

A printing apparatus ( 100 ) for printing digital images onto a photosensitive medium ( 140 ) employing, for exposure energy, a light source ( 20 ) that uses various arrays of LEDs ( 32 ). The printing apparatus ( 100 ) may form the print image using sequential modulation, one color at a time, or by applying all colors simultaneously. Arrangements of discrete LEDs ( 32 ) may include high-intensity devices configured with collector cones ( 41 ) arranged as a multicone structure ( 141 ), with parabolic reflectors ( 65 ), or collimating lenses ( 36 ). Large area LEDs ( 46 ) may alternately be used, arranged on an angled mounting surface ( 64 ), for example.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. 09/794,669, filed Feb. 27, 2001, entitled METHOD AND APPARATUSFOR PRINTING HIGH RESOLUTION IMAGES USING MULTIPLE REFLECTIVE SPATIALLIGHT MODULATORS, by Ramanujan et al., and U.S. application Ser. No.09/976,171, filed Oct. 12, 2001, entitled A TWO LEVEL IMAGE WRITER, byRoddy, et al., the disclosures of which are incorporated herein.

FIELD OF THE INVENTION

The apparatus and method of the present invention generally relate towriters for forming an image from digital data onto a photosensitivemedium and more particularly relate to an improved light sourceemploying LEDs for recording an image onto a photosensitive medium.

BACKGROUND OF THE INVENTION

With the advent of digital imaging, a number of different writingtechnologies have been employed for imaging onto photosensitive mediasuch as photographic paper and motion picture film. Early printersadapted cathode ray tubes (CRTs) for providing exposure energy onto thephotosensitive medium. In a CRT-based printer, the digital image data isused to modulate the CRT, providing exposure energy by scanning anelectron beam of variable intensity along a phosphorescent screen. WhileCRT-based imaging provided a suitable solution for some imagingapplications, high cost and relatively slow speeds, as well asconstraints on resolution and contrast limit the usefulness of thisapproach.

Alternative writers employ lasers, such as the laser-based writingengine disclosed in U.S. Pat. No. 4,728,965. With this type of imaging,red, green, and blue lasers provide the exposure energy. Digital data isused to modulate laser intensity as the beam is scanned by a rotatingpolygon onto the imaging plane. Unfortunately, as with CRT printers,laser based systems tend to be expensive, particularly since the cost ofblue and green lasers remains high. Additionally, compact lasers havingsufficiently low noise levels and the stable output behavior necessaryfor accurate reproduction of an image without unwanted artifacts are notwidely available. While lasers provide advantages for high-powerapplications, they are not well suited to the reciprocitycharacteristics of conventional photographic film and paper media. Thus,special media is often required for imaging using laser exposure energy.

The speed, cost, and performance problems of existing digital imagingsystems limit the utility of such systems with some types ofphotosensitive media. These problems are particularly pronounced forhigh speed raster printing applications, such as those needed whenprinting motion picture film. CRT printers, for example, requireexposure durations lasting as long as a few minutes per frame.Commercially available raster laser print systems are faster, but stillrequire 3-10 seconds per frame. For digital mastering of a full lengthfeature film to be commercially feasible, however, print speeds of atleast two frames per second are required. Writing film at real-timespeeds would require significantly better throughput, at approximately24-30 frames per second. As the figures given here indicate, this 24-30frames per second speed appears to be out of reach of conventional CRTand laser technologies using raster imaging methods.

Two-dimensional spatial light modulators, while originally developed forprojectors and displays, also show promise for writing applications.Unlike the slower, raster-scanned energy sources of CRT andlaser-polygon devices, spatial light modulators provide exposure energyfor a complete image frame at a time. Essentially a two-dimensionalarray of light-valve elements, with each element corresponding to apixel, the spatial light modulator operates by selectively reflecting orchanging the polarization state of each image pixel. Standard types ofspatial light modulators include digital micromirror devices (DMDs) andliquid crystal devices (LCDs).

DMD solutions, such as that shown in U.S. Pat. No. 5,461,411 offeradvantages such as longer exposure times compared to laser/polygonwriters. This helps to alleviate reciprocity problems associated withphotosensitive media during short periods of light exposure. However,DMD technology is expensive and is not widely available. Furthermore,DMDs are not currently available at high enough resolutions for printerapplications and are not easily scaleable to higher resolution.

The LCD type of spatial light modulator modulates by selectivelyaltering the polarization state of each image pixel. Types of LCDinclude transmissive and reflective. Both types of device have been usedin imaging systems. For example, U.S. Pat. Nos. 5,652,661 and 5,701,185disclose printing apparatus that form images using transmissive LCDs.However, there are several drawbacks to the use of conventionaltransmissive LCD technology. Because of the space required for circuittraces and components, transmissive LCD modulators generally havereduced aperture ratios. Transmissive field-effect-transistors(TFT)-on-glass technology does not provide sufficient pixel to pixeluniformity needed for many printing and film recording applications.Furthermore, the large footprints of transmissive LCD devices, needed inorder to provide large numbers of pixels for improved resolution, proveto be unwieldy as part of an optical system designed for printing orfilm recording applications. As a result, most LCD printers usingtransmissive technology are constrained to either low resolution orsmall print sizes.

In contrast, reflective LCD modulators provide superior performance andsignificantly reduce the cost of the printing system. Exposure times forindividual pixels shift from tens of nanoseconds to tens ofmilliseconds, a million-fold increase. This increased exposure time,along with an increased aperture size for each pixel, allows a modestincrease in writer throughput to two frames per second or better,without the media reciprocity problems otherwise caused by shortexposure times.

Both transmissive and reflective types of LCDs present limitations.However, particularly with reflective LCDs, device performance continuesto improve, making it advantageous to employ reflective LCDs in printerapplications.

LCDs can modulate light from any of a number of sources. Conventionalprinters employ lamps as light sources. However, although lamps canprovide output power levels sufficient for high speed printing, they dohave some inherent limitations, such as high levels of heat and IRgeneration. Additionally, lamps cannot be switched on and off at ratessufficient for high speed print applications. For reasons cited above,lasers continue to be less desirable for writing to photosensitivemedia. In addition, laser sources exhibit undesirable coherent lighteffects, such as speckle. Light emitting diodes (LEDs) have been used,with some degree of success, in printers, however, there is considerableroom for improvement.

It is well recognized in the imaging arts that LEDs are not designedwith writing applications in mind. Conventionally manufactured for usein display applications with visible light, LEDs are designed to providemodest levels of brightness with high angular divergence. Since they arecharacterized primarily for display functions, LEDs are specified bymanufacturers for brightness and other photometric characteristics thatrelate to human eye response. Photometric ratings of LED light sources,usually given in lumens or candelas, give very little useful informationrelated to suitability for film exposure.

It is the radiometric, rather than photometric, characteristics of thelight source that are of most interest when writing onto photosensitivemedia. Writing wavelengths, for example, need not be in the visiblerange; there can be advantages in using light with very short,near-ultraviolet wavelengths or in longer infrared wavelengths. Writingspeed, for example, is a function of radiance (typically expressed inwatts per square centimeter-steradian), rather than of brightness. Thehigh angular divergence of light from LEDs, beneficial for displaypurposes, is detrimental for writing purposes, where a narrow emissionangle and small emission area works best for achieving high resolutionand high speed. Specifications of dominant wavelength, conventionallyprovided for high-brightness display LEDs, can be misleading from theperspective of characteristics needed for writing to photosensitivemedia. The actual peak emission wavelength can vary substantially fromthe dominant wavelength that is perceived by the human eye.

Photosensitive Media Characteristics

As is described above, LEDs are largely designed for visibility to thehuman eye, for use in a variety of display applications. Design fordisplay visibility, however, often conflicts with requirements forexposure of photosensitive media. Referring to FIG. 3, there is shown arepresentative graph of spectral sensitivity versus wavelength per colorlayer for a type of color internegative film used in motion pictureprinting. The vertical scale is a log scale. Thus, a 0 value correspondsto an exposure level of 1 erg/cm²; a value of 1 corresponds to anexposure level of 0.1 erg/cm²; a value of −1 corresponds to an exposurelevel of 10 erg/cm². For comparison, overlaid onto graphs for spectralsensitivity of this photosensitive medium is a normalized curve showingthe photopic response of the human eye. As the graph of FIG. 3indicates, there can be substantial differences between the response ofa photosensitive medium over a range of wavelengths and the response ofthe human eye. Certainly, the human eye has a heightened response tosubtle changes of color intensity and hue in some parts of the spectrum.Film sensitivity, however, is much more pronounced for differentexposure wavelengths. As shown in FIG. 3, for example, sensitivity forthe red layer is roughly 1/100 that of the blue layer.

The color response characteristics of photosensitive media often differfrom response of the human eye within the same region of the spectrum.For example, the ideal blue wavelength for film exposure isapproximately 450 nm, a value near the peak of the blue curve and nearthe minimum value of the green curve. Note that, for visibility to thehuman eye, however, a value of 480-490 nm would be much preferred. Withthe film sensitivity shown in FIG. 3, however, a 490 nm exposure wouldnot be appropriate, since it would affect both blue and green layers tosomewhat the same degree. Similarly, for the red layer, an optimalwavelength for the film would be in the 685-695 nm region. However, thiswavelength may be difficult to perceive, being very near the edge of thevisible spectrum. Note that a red LED, with a dominant wavelengthtypically near 625 nm, is optimal for traffic light or other visibleuses. However, for film exposure, almost 4 times the amount of light isneeded at this wavelength than would be required at 690 nm. Thus, it canbe seen that the sensitivity of photosensitive media to wavelengths mayeven conflict with human eye response, for which LEDs are primarilydesigned.

Clearly, there are significant differences between what is required ofthe LED as a light source for display versus what is required as anexposure source for imaging onto photosensitive media. Because LEDs havebeen primarily developed for display applications, special techniquesare required to adapt these devices to a high-speed writing apparatus.

LED Composition and Characteristics

FIGS. 1 a, 1 b, and 1 c show, from top, side, and cross-sectionalperspective views respectively, the construction of a conventionalencapsulated discrete LED 32 used in indicator lamp and other displayapplications. Referring to FIG. 1 a, a bond wire 27 is connected to anelectrode 30 on an LED chip 25. For a typical device, discrete LED 32 isapproximately 200 to 250 um square, with electrode 30 having a diameterin the 120 to 150 um range. Necessarily, electrode 30 covers asubstantial portion of the light emitting area of discrete LED 32 in thedesired emission direction. Referring to FIG. 1 b, in which LED chip 25is soldered to a substrate 29, the preferred emission direction for LEDlight is shown. However, as indicated in FIG. 1 b, a considerable amountof light is emitted in undesirable directions, from the edges of LEDchip 25. This is due, for example, to the reflective effects of thesolder and to some reflectivity of electrode 30 itself. Bond wire 27also blocks some amount of light. It is difficult to collect and uselight emitted from these edges. In fact, because of the position ofelectrode 30, only a small amount of light is actually emitted along thepreferred axis. The optical axis itself can be darker than off-axisareas.

Referring to the cross-section of discrete LED 32 shown in FIG. 1 c, LEDchip 25 is positioned within a reflector cup 24 which helps to collectsome of the light emitted in an undesirable side direction and directthis light vertically in the desired direction. The conventionaldiscrete LED 32 is encapsulated in an epoxy dome lens 28, which helpssomewhat to direct light in the desired direction. Drive current issupplied from an anode lead 21, through a thin gold bond wire 27, thenthrough LED chip 25, to a cathode lead 23. For cooling, convection andheat radiation are negligible. Instead, heat generated within discreteLED 32 must be conducted from discrete LED 32 by cathode lead 23. Highthermal resistance reduces overall LED power and device lifetime.

The conventional design approach used for discrete LED 32, as shown inFIGS. 1 a, 1 b, and 1 c, is acceptable for many display applications.However, as is noted in the above description, due to sensitometricresponse characteristics of photosensitive media and to drawbacks ofconventional discrete LED 32 design, the conventional approach isrelatively inefficient and is not well-suited to writing applications.Some improvements can be made in the design of discrete LED 32 devices.For example, referring to FIGS. 2 a and 2 b, there are shown top andside views, respectively, of an improved design for discrete LED 32.Here, two electrodes 30 are positioned in diagonal corners of LED chip25, thereby eliminating the on-axis dark spot of convention discreteLEDs 32 and allowing increased light emission in the desired directionof the optical axis. In addition, drive current is more uniformlydistributed within LED chip 25 with the arrangement of FIGS. 2 a and 2b. However, thermal build-up remains a problem that is not sufficientlyalleviated with the design solution of FIGS. 2 a and 2 b.

Solutions for Grouping LEDs

One method for obtaining higher levels of exposure energy from discreteLEDs 32 is to group together multiple discrete LEDs 32. The relativelysmall size of discrete LED 32 components makes this approach feasiblewithin some limits. Referring to FIG. 4 a, there is shown one example ofa 3×3 array of discrete LEDs 32, in an arrangement of colors. Such anarray of discrete LEDs 32, assembled on an LED mount plate 31, can bedeployed to provide increased brightness where it is advantageous to usediscrete LED 32 sources. As FIG. 4 a shows, it may be advantageous toarrange discrete LEDs 32 in various patterns and to have relatively morediscrete LEDs 32 of some colors, based on the spectral sensitivity ofthe photosensitive medium being exposed.

In a typical writing apparatus, the acceptance cone angle of theillumination system determines within what area discrete LEDs 32 can beplaced on LED mount plate 31. In practice, suitable illumination opticstypically have an acceptance cone angle of about f/4. With conventionallens components and design approaches, such as using a 25 mm diameterachromatic lens with a 100 mm focal length, for example, this wouldconstrain the available area, requiring LEDs to be positioned withinapproximately a 1 inch square.

Because of simple light cone geometry, there are some limitations tolight efficiency in providing multiple discrete LEDs 32 within a narrowillumination aperture, as shown with respect to FIGS. 5 a and 5 b. InFIG. 5 b, discrete LED 32 emits light having an overall LED divergenceangle 33 that exceeds an optics acceptance cone angle 34 of acollimating lens 36 for the illumination optics. A waste light portion35 of emitted light from discrete LED 32 lies outside optics acceptancecone angle 34 and is, therefore, unusable within the printing apparatus.Where multiple discrete LEDs 32 are deployed, even where the emissioncone angle is smaller than the optics acceptance cone angle, there canstill be considerable unused light, as shown in FIG. 5 a. Here, discreteLEDs 32 on the outskirts of LED mount plate 31 have an increased amountof unused light, making a larger contribution to waste portion 35 thando more centrally located discrete LEDs 32. Thus, simply increasingdiscrete LED 32 density has its limitations for providing increasedexposure energy. Solutions using smaller discrete LED 32 components mayprovide a modest increase in exposure energy, but are also subject tosimilar inefficiencies due to angular divergence.

An alternate approach for providing sufficient exposure energy is theuse of large area LED devices having patterned electrodes, as shown inFIGS. 7 a and 7 b. This approach provides, from a relatively smallillumination area, the equivalent energy of an array of discrete LEDs32. Commercial versions of compact large area LED devices include LuxeonStar devices available from Lumileds Lighting, LLC, located in San Jose,Calif. or similar components available from Cree, Inc., located inDurham, N.C., for example. Referring to FIG. 7 a, one type of large areaLED device, a patterned electrode LED 134, has a patterned electrode 40that spreads current uniformly through LED chip 25 without substantiallyobstructing emission in the desired direction. FIG. 7 b shows a sideview of patterned electrode LED 134. By way of comparison, LED chip 25used in patterned electrode LED 134 is approximately 2 mm on an edge,about ten times the length of a standard LED chip 25 as used in aconventional discrete LED 32. However, patterned electrode LED 134 isstill small enough to be sufficiently collimated by a single lens. Acustom lenslet array is not required.

FIG. 8 a shows, in more detail, how patterned electrode LED 134 such asthe Luxeon Star device is constructed. A large area LED 46 is mounted ona metal heatsink 48 and is connected externally by electrical leads 43and internally by bond wires 27. The device is covered by a clearplastic lens 128. As shown in FIG. 8 b, auxiliary collector optics canbe added to increase light intensity from patterned electrode LED 134. Acollector cone 41, such as a plastic molded component, serves as a lightguide and provides both collimating and reflective optics for directingthe emitted light. Collector cone 41 comprises an integrally moldedcollimating lens 36 to aid in collimating emitted light received at itsinput end to provide narrower angle emission. Additionally, collectorcone 41, as a prism structure, acts as a guide element using totalinternal reflection (TIR) to help redirect the wider angle emissions. Itcan be seen that the patterned electrode large area LED goes a long waytowards providing a light source for film imaging applications,providing relatively high radiance, high power, small emission angle,and small source area, and having reduced tendency for thermal buildupand absence of a “dead spot” on axis.

While solutions such as clustering LEDs and combining patternedelectrode LED 134 with collector cone 41 provide some improvement forachieving high levels of exposure energy from LED sources, it can beappreciated that significant difficulties remain. In order to providesufficient exposure energy for printing at efficient speeds, even moreintense illumination energy is needed. At the same time, however, thisenergy must be emitted from a small source, from within a limited area,and at low divergence angles.

Conventional approaches currently allow writing speeds of up to about 1frame/second. Commercial viability, however, demands speeds approaching24 frames/second. Because increases in exposure energy translatedirectly to potential increases in writing speed, even incrementalimprovements in providing increased exposure energy can be beneficial,provided that the necessary area limitations and divergence angleconstraints are met.

Thus, there is a need for an improved printing apparatus capable of highspeed printing onto photosensitive media and utilizing high-intensityLED illumination.

SUMMARY OF THE INVENTION

The present invention provides an improved light source for providing anillumination beam along an illumination axis, comprising:

(a) a plurality of large area LEDs, mounted on a base, for generatingemitted light;

(b) a collector structure for collecting the emitted light from theplurality of large area LEDs to form the illumination beam, thecollector structure comprising:

-   -   for each LED, a light guide for collecting, at an input, the        emitted light received from the LED and directing the emitted        light to an output, along the illumination axis, the light guide        comprising:        -   (i) a collimating element near the input for collimating a            portion of the emitted light to direct collimated light in            the direction of the illumination axis; and        -   (ii) a guide element for directing some of the other emitted            light at the input to the output and along the illumination            axis;    -   wherein, the output of at least a first light guide has at least        one flat edge for butting against the output of at least a        second light guide.

It is a feature of the present invention that it deploys multiple LEDshaving different wavelengths, based on sensitivity characteristics ofthe photosensitive medium. The apparatus and methods of the presentinvention take advantage of some of the features and affordability ofLED components, while configuring these devices in suitable ways forwriting to film and other photosensitive media. For example, the presentinvention takes advantage of the narrow-band emission, relatively highefficiency, fast switching rates, relatively low cost, small size, andminimal heat and IR emission of LEDs.

It is an advantage of the present invention that it increases the amountof light energy available for printing when using LED sources using anumber of arrangements and optical support components.

It is a further advantage of the present invention that it allowscompact arrangements of LEDs used as exposure light sources.

These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIGS. 1 a, 1 b, and 1 c show fabrication details for conventionaldiscrete LEDs, such as LEDs used for display;

FIGS. 2 a and 2 b show, in top and side views respectively, fabricationdetails for an improved LED design using corner electrodes;

FIG. 3 gives a graph of representative sensitivity characteristics for atypical color internegative film, along with an overlaid normalizedgraph showing the photopic response of the human eye;

FIG. 4 a shows a front view of a 3×3 array of LEDs containing five red,two green and two blue LEDs;

FIG. 4 b shows a front view of an array having multiple LEDs packed in ahoneycomb arrangement for maximum packing density;

FIG. 5 a is a schematic drawing that shows an LED array where some lightfrom the outermost LEDs is not collected by the lens;

FIG. 5 b is a schematic drawing that shows an LED with wide emissionangle that exceeds the acceptance cone angle of the lens;

FIG. 6 a is a top view of an LED array comprising an arrangement ofindividual LED elements on a common baseplate;

FIG. 6 b is a side view of the LED array of FIG. 6 a, showing a customlenslet array for collimating light from each LED element;

FIGS. 7 a and 7 b show top and side views, respectively, of an improvedLED having interdigitated electrodes;

FIG. 8 a shows a cross sectional view of an improved LED;

FIG. 8 b shows a collimator/reflector structure used with the LED ofFIG. 8 a;

FIG. 9 shows a multicolor LED array having a single LED with acollimator/reflector structure, surrounded by discrete LEDs of differentcolor;

FIG. 10 a shows a light source configuration with four LEDs of a singlecolor, each having a modified collimator/reflector structure;

FIG. 10 b shows, from a front view, a light source configuration withfour LEDs of a single color, each having a collimator/reflectorstructure of the present invention;

FIG. 10 c shows, from a front view, a light source configuration withfour LEDs of different colors, using the collimator/reflector structureof the present invention;

FIG. 11 a shows four improved LEDs, mounted on a thermoelectric cooler,with an aperture plate containing collimator lenses;

FIG. 11 b shows a light source configuration with four LEDs of a singlecolor, each having a modified collimator/reflector structure, mounted ona thermoelectric cooler;

FIG. 12 shows an alternate arrangement in which a light source has redand green LEDs in a four leaf clover structure similar to that shown inFIG. 10 a, with additional blue LEDs added to form a multicolor array;

FIG. 13 is a schematic view of a printing apparatus according to thepresent invention, using multiple illumination sources combined througha dichroic mirror;

FIG. 14 a is a side view showing a structure in which multiple LEDs aremounted on an angled surface, thereby directing light toward the centerof a collimating lens;

FIG. 14 b is a perspective view of the device having LEDs mounted on anangled surface, as shown in FIG. 14 a, where the LEDs include thecollimator/reflector structure shown in FIG. 8 b;

FIG. 15 shows an LED configured with a collimating lens and a parabolicreflector to aid in light collection and collimation;

FIGS. 16 a, 16 b, 16 c, and 16 d show multiple views of a writingapparatus of the present invention that employs three spatial lightmodulators;

FIG. 17 is a schematic diagram of an alternate printing apparatus havingthree separate writing heads, one for each color; and

FIG. 18 is a schematic view of a printing apparatus according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

FIG. 18 schematically illustrates a printer apparatus 100 for printingonto a photosensitive medium 160, such as a motion picture print film.Printer apparatus 100 comprises an optics assembly 10 and a mediahandling subsystem 212. Media handling subsystem 212 comprises a filmsupply 202, an exposure section 204, and a film storage unit 208. Acontrol logic processor 210 accepts and processes image data for printerapparatus 100 and controls the overall operation of optics assembly 10and media handling subsystem 212 components. The operation of printerapparatus 100 follows the general pattern used for other printer types.To print, an unexposed section of a photosensitive medium 160 isadvanced from film supply 202 into exposure section 204. Optics assembly10 cooperates with control logic processor 210 to print image data ontophotosensitive medium 160. The exposed section of photosensitive medium160 is then ready for processing in order to develop the image.

Within optics assembly 10, are the components for forming the image andfocusing the image onto photosensitive medium 160. A light source 20directs monochromatic light having one of a set of possible colors to apolarizing beamsplitter 16 which directs light having suitablepolarization state to a spatial light modulator 14, an LCD in apreferred embodiment. Spatial light modulator forms the image bymodulating the polarization state of incident light and reflects themodulated light, which is then focused onto exposure section 204 byfocusing optics 18. Uniformizing optics 12 homogenize light from lightsource 20 to provide a uniform field for modulation by spatial lightmodulator 14.

For the present invention, light source 20 uses a combination of LEDsources in an array, arranged to provide sufficient exposure energy forhigh-speed processing of photosensitive medium 160. As is shown in thevarious embodiments described herein, the LED sources at light source 20can be all the same color. Alternately, LED sources at light source 20can be of different colors, separately energized to expose successiveframes of photosensitive medium 160. Sensitivity of photosensitivemedium 160 to various wavelengths, as shown in FIG. 3, is an importantfactor in light source 20 design. As is noted in the backgrounddiscussion above, light source 20 has an effective illumination area ofabout one square inch. An important goal for high-speed performance isto provide, from light source 20, maximum radiance for exposure ofphotosensitive medium 160.

Referring to FIG. 13, there is shown an alternate design approach forprinting apparatus 100 for sequential imaging in which, sharing the useof a single modulator for all colors, light source 20 may utilize morethan one array of LED sources, optically along a common illuminationaxis. During a first periodically repeating time interval, a red LEDarray 60 provides source illumination along an illumination axis I.During a second periodically repeating time interval, green light from agreen and blue LED array 61 provides source illumination. Similarly,during a third periodically repeating time interval, blue light fromgreen and blue LED array 61 provides source illumination. A dichroicmirror 62 combines light from green and blue LED array 61 and red LEDarray 60 and directs the light through collimating lens 36 anduniformizing optics 45 along illumination axis I. Additional collimatinglenses 36 are provided for each LED array 60 and 61. A telecentriccondenser lens 70 relays the illumination through a polarizer 52 to apolarizing beamsplitter 81. Light reflected from polarizing beamsplitter81, having the proper polarization state, is then modulated at a spatiallight modulator 91, passes through polarizing beamsplitter 81, and isoutput along an output axis O, through an analyzer 63 to a print lensassembly 110 which focuses the image onto a photosensitive medium 140.

Dichroic mirror 62 can help to adapt the characteristics of the LEDsources to the specific film or other photosensitive medium 140. Forexample, dichroic mirror 62 can be designed to clip the shorterwavelengths of the red emission to minimize unwanted green exposure.This would obviate the need for an optional filter.

The design of FIG. 13 admits a number of variations, as would befamiliar to those skilled in the optical design arts. Uniformizingoptics 45 could comprise a lenslet array, an optical tunnel, anintegrator bar, or even an integrating sphere, for example. Polarizingbeamsplitter 81, shown schematically as a MacNeille prism, couldalternately be embodied as a wire grid beamsplitter. Analyzer 63 may notbe necessary if the contrast ratio is acceptable. Further, any of theimproved embodiments of light source 20, described herein below, couldbe employed in printer apparatus 100 as shown in FIG. 13, as well aswith those printer apparatus 100 embodiments shown in FIGS. 16 a-16 d,17, or 18.

Referring to FIGS. 16 a, 16 b, 16 c, and 16 d, there are shown schematicand perspective views of printing apparatus 100 in an embodiment using aseparate light source and modulator for each color. Referringparticularly to FIGS. 16 a, 16 b, and 16 c, light sources 20, 22, and 26direct light (typically red, green, and blue light) through uniformizingoptics 45, 49, and 47; through telecentric condenser lens 70, 72, 71;through polarizers 53, 55, and 57; through folding mirrors 73, 75, 77;through polarizing beamsplitters 80, 84, and 82; to reflective spatiallight modulators 90, 97, and 95 respectively. Modulated light istransmitted through polarizing beamsplitters 80, 84, and 82 to an X-cubecombiner 86, which combines the separately modulated color beams alongan output axis for writing. FIG. 16 d shows one example for spatialpositioning of these illumination and modulation components withinprinting apparatus 100.

In the arrangement shown in FIGS. 16 a, 16 b, 16 c, and 16 d, lightsources 20, 22, and 26 each comprise an LED array configured with one ofthe embodiments disclosed herein. Spatial light modulators 90, 97, and95 are reflective LCDs. The arrangement of FIGS. 16 a, 16 b, 16 c, and16 d allows simultaneous exposure of photosensitive medium 140. Becausethis design allows maximum power for each color and simultaneous writingof colors, writing speed is maximized when using this arrangement.

Referring to FIG. 17, there is shown yet another embodiment of printingapparatus 100 wherein a separate single-color writer module 66 providesexposure to print lens 110 for each color. Each single-color writermodule 66 could have the structure of optics assembly 10 in FIG. 18, forexample. With the arrangement shown in FIG. 17, photosensitive medium140 is indexed from one position to the next in order to expose allcolors for each image frame. All three single-color writer modules 66image simultaneously for successive image frames. With this arrangement,for example, one color writer module 66 writes the red component of animage frame. The next color writer module 66 in sequence writes thegreen component. The last color writer module 66 writes the bluecomponent. Using this arrangement, imaging optics can be optimized forspecific colors, minimizing chromatic aberrations.

Basic Embodiment—Hexagonal Arrangement of Discrete LEDs 32

Referring to FIG. 4 b, there is shown, from a front view, a firstembodiment for light source 20 in the present invention. LED mount plate31 serves as a support for a number of discrete LEDs 32, arranged in ahexagonal or honeycomb pattern. The hexagonal pattern allows the closestpossible packing of discrete LEDs 32 in the same plane. In the bestarrangement, discrete LEDs 32 would be substantially within the boundsof a 1-inch square source area, as shown. In the arrangement of FIG. 4b, twenty-two discrete LEDs 32 are provided: twelve red, nine green, andone blue discrete LED 32. The number of different color devicescorresponds closely to sensitivity differences in photosensitive medium160, as suggested in FIG. 3. Important factors in determining the numberand type of discrete LEDs 32 of each color include emitted wavelength,sensitivity of photosensitive medium 160, exposure time considerations,and LED output power. For example, for a specific photosensitive medium160, needed exposure levels for red, green, and blue light,respectively, may be 10, 6, and 2 ergs per square centimeter. As anexample, where discrete LEDs 32 have equal output power and suitablewavelength characteristics, ten red, six green, and two blue discreteLEDs 32 would be needed for an arrangement similar to that shown in FIG.4 b, assuming equal exposure times.

Single-Substrate Embodiment

Alternative solutions for increasing exposure energy output from LEDsources include using more compact fabrication. Referring to theschematic plan view and side views of FIGS. 6 a and 6 b, respectively,an LED array 130 has been fabricated by mounting LED chips 25 onto asubstrate 131, leaving an appropriate interchip gap for bond wirerouting. To fabricate LED array 130, LED chips 25 were mounted andwired, then LED array 130 coated with an epoxy or other suitableprotectant. The example of FIG. 6 a shows fabrication of a 14×14 chipLED array 130. To provide collimation, a custom lenslet array 38 can bealigned to LED chips 25 in LED array 130 and can then be permanentlyattached. While solutions of the type shown in FIGS. 6 a and 6 b havebeen attempted, results can be disappointing, due to high fabricationcosts and significant internal light loss.

Clustered Embodiment with Single Collector Cone 41

Referring to FIG. 9, there is shown, in a front view, another alternateembodiment of light source 20 in which a multicolor array is formedusing a plurality of LEDs. At the center, a single green large area LED46 is provided with collector cone 41 as is shown in FIG. 8 b.Additional discrete LEDs 32 are also provided on LED mount plate 31.Collector cone 41 is notched to accommodate closely packed, neighboringred and blue discrete LEDs 32. The arrangement of FIG. 9 is suitablebecause large area LEDs 46 is several times as bright as conventionaldiscrete LEDs 32, particularly when combined with collector cone 41.

Clustered Embodiments with Multiple Collector Cones 41

Referring to FIG. 10 a, there is shown an arrangement of collector cones41 for use in light source 20. As this arrangement shows, none ofcollector cones 41 and none of large area LEDs 46 are centered onoptical axis O. Thus, this arrangement is relatively inefficient, with adark spot centered on optical axis O.

Referring to FIG. 10 b, there is shown, from a front view, an improvedarrangement having multiple large area LEDs 46 of the same color in amulticone structure 141. Unlike the arrangement with separate collectorcones 41 as shown in FIG. 10 a, multicone structure 141 directs theemitted light more closely to optical axis O.

Multicone structure 141 is formed by fitting a plurality of conesegments 101 together. Cone segments 101 can be cut, molded, orotherwise shaped in order to provide a compact arrangement in whichoutput sections of cone segments 101 are butted against each other. Thisarrangement maximizes the contribution of each large area LED 46 toon-axis illumination.

The example of FIGS. 10 a and 10 b provide a single color. For sometypes of imaging apparatus, individual color light sources 20 areappropriate. Various alternate arrangements are possible. For example,the embodiment of FIG. 10 c shows an arrangement in which differentcolors can be deployed within a cluster. Here, red, green, and bluelarge area LEDs 46 are provided with multicone structure 141.Alternatively, in FIG. 12, blue discrete LEDs 32 are placed nearmulticone structure 141 for red and green large area LEDs 46 to provideblue wavelengths.

It must be emphasized that, for printing applications, a key designconsideration is the relative sensitivity of the photosensitive film orother medium at various wavelengths. With the motion picture mediumhaving the response shown in FIG. 3, for example, an ideal redwavelength for the red layer would be 690 nm. Typical red display LEDs,on the other hand, have peak wavelengths at 625-640 nm. Based on therelative sensitivity from FIG. 3, almost four times the intensity wouldbe needed at 625 nm than is needed at 690 nm. This does not preclude theuse of red LEDs having the lower wavelength value; instead, thisrelationship simply determines how many red large area LEDs 46 of thestandard type are required.

Embodiments Allowing More Compact Packaging of Large Area LEDs 46

Reducing generated heat extends both LED lifetime and allows operationat increased power and brightness levels. Referring to FIG. 11 a, thereis shown, from a side view, an embodiment of light source 20 that allowseven denser packing and larger LEDs. Large area LEDs 46, such as theLuxeon devices noted above, are mounted on LED mount plate 31. Toincrease both power output and lifetime, a thermoelectric cooler 51 isplaced in contact with LED mount plate 31. A heatsink 56 is alsoprovided to dissipate the generated heat. For each large area LED 46, anaperture plate 50 mounts a corresponding discrete collimating lens 36.Note that FIG. 11 a shows a side view; from a top view, four or morelarge area LEDs 46 could be clustered together as with the embodimentsof FIGS. 10 a and 10 b.

Referring to FIG. 11 b, there is shown another optional embodiment, withcollector cones 41 used together with large area LEDs 46 for collimationand collection of emitted light. As is indicated in FIG. 11 b, collectorcones 41 may be shaped to fit closely against each other.

Referring to FIGS. 14 a and 14 b, there is shown, in side view andperspective view, respectively, an embodiment of light source 20 inwhich large area LEDs 46 are deployed on an angled mounting surface 64.Here, angled mounting surface 64 is generally of a concave shape withrespect to the illumination axis I. Optionally, some degree of curvaturecould also be provided to shape angled mounting surface 64. Thearrangement of FIGS. 14 a and 14 b directs light toward a point F thatrepresents an ideal aperture for collimating lens 36 along theillumination optics axis.

While collector cones 41 provide one type of useful mechanism fordirecting light from large area LEDs 46, other types of structure couldbe used. For example, appropriately shaped glass prisms could beemployed, each shaped to include a collimating input element and to usetotal internal reflection for directing light to an output.

Embodiment to Improve Efficiency of Light Collection

Referring to FIG. 15, there is shown an arrangement for improved lightefficiency. Large area LED 46 is located at the focus of a parabolicreflector 65 that collimates widely divergent rays. Collimating lens 36collimates the less divergent rays. This design is an improvement overcollector cone 41 of FIG. 8 b which simply sends the widely divergentrays forward, without collimation. Light rays exiting collector cone 41at an angle greater than an f/4 acceptance cone, approximately 7degrees, are lost and do not contribute to exposure.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention as described above, and as noted in the appended claims, by aperson of ordinary skill in the art without departing from the scope ofthe invention.

Thus, what is provided is an improved apparatus and method for printingonto a photosensitive medium using LED light sources.

Parts List

-   10. Optics assembly-   12. Uniformizing optics-   14. Spatial light modulator-   16. Polarizing beamsplitter-   18. Focusing optics-   20. Light source-   21. Anode lead-   22. Light source-   23. Cathode lead-   24. Reflector cup-   25. LED chip-   26. Light source-   27. Bond wire-   28. Epoxy dome lens-   29. Substrate-   30. Electrode-   31. LED mount plate-   32. Discrete LED-   33. LED divergence angle-   34. Optics acceptance cone angle-   35. Waste light portion-   36. Collimating lens-   38. Lenslet array (collimator)-   40. Patterned electrode-   41. Collector cone-   43. Electrical leads-   45. Uniformizing optics-   46. Large area LED-   47. Uniformizing optics-   48. Heatsink-   49. Uniformizing optics-   50. Aperture plate-   51. Thermo-electric cooler-   52. Polarizer-   53. Polarizer-   55. Polarizer-   56. Heatsink-   57. Polarizer-   60. Red LED array-   61. Green and blue LED array-   62. Dichroic mirror-   63. Analyzer-   64. Angled mounting surface-   65. Parabolic reflector-   66. Single color writer module-   70. Telecentric condenser lens-   71. Telecentric condenser lens-   72. Telecentric condenser lens-   73. Folding mirror-   75. Folding mirror-   77. Folding mirror-   80. Polarizing beamsplitter-   81. Polarizing beamsplitter-   82. Polarizing beamsplitter-   84. Polarizing beamsplitter-   86. X-cube combiner-   90. Spatial light modulator-   91. Spatial light modulator (SLM)-   95. Spatial light modulator-   97. Spatial light modulator-   100. Printer apparatus-   101. Cone segments-   110. Print lens assembly-   128. Clear plastic lens-   130. LED array-   131. Substrate-   134. Patterned electrode LED-   140. Photosensitive medium-   160. Photosensitive medium-   141. Multicone structure-   202. Film supply-   204. Exposure section-   208. Film storage unit-   210. Control logic processor-   212. Media handling subsystem

1. In a printing apparatus for recording images onto photosensitivemedium, an improved light source for directing, along an illuminationaxis, light having a first, second, or third wavelength, the lightsource comprising: (a) an array of LEDs hexagonally arranged forminimum, substantially equal spacing between each LED and each of itsimmediate neighbor LEDs, any one of said LEDs having either a first or asecond or a third wavelength, the hexagonally arranged LEDs comprisingrows of LEDs with LEDs of adjacent rows being mutually partiallypositioned within recesses formed between adjacent LEDs in a respectiverow; (b) a logic controller for enabling LEDs within said arrayaccording to said first, second, or third wavelength; (c) collectoroptics for collecting emitted light from said at least one firstwavelength LED and directing said emitted light from said at least onefirst wavelength LED along the illumination axis; and wherein saidcollector optics comprise a collimating element and a light guideemploying total internal reflection for conditioning light emitted fromsaid at least one first wavelength LED.
 2. In a printing apparatus forrecording images onto a photosensitive medium, an improved light sourcefor directing, along an illumination axis, light having a first, second,or third wavelength, the apparatus comprising: (a) at least one firstwavelength LED, positioned on a support element; (b) collector opticsfor collecting emitted light from said at least one first wavelength LEDand directing said emitted light from said at least one first wavelengthLED along the illumination axis; wherein said collector optics comprisea collimating element and a light guide employing total internalreflection for conditioning light emitted from said at least one firstwavelength LED; (c) at least one second wavelength LED, spaced apartfrom said at least one first wavelength LED, positioned on said supportelement and arranged to direct light along the illumination axis; and(d) at least one third wavelength LED, spaced apart from said at leastone first wavelength LED, positioned on said support element andarranged to direct light along the illumination axis.
 3. An improvedlight source according to claim 2 wherein said at least one firstwavelength LED is green.
 4. An improved light source according to claim2 wherein said support element is generally concave with respect to saidillumination axis, such that said at least one second and said at leastone third wavelength LEDs are angularly inclined towards saidillumination axis.
 5. An improved light source according to claim 2wherein said collector optics comprise a curved reflector for directingsaid light from said at least one first wavelength LED.
 6. An improvedlight source according to claim 5 wherein said curved reflector issubstantially parabolic.
 7. A printing apparatus for recording imagesonto photosensitive medium, comprising: (a) at least one firstwavelength LED, mounted on a first support element; (b) first collectoroptics for collecting emitted light from said at least one firstwavelength LED and directing said emitted light from said at least onefirst wavelength LED through a dichroic combiner and along anillumination axis, wherein said first collector optics comprises a lightguide having an integral collimating element and employing totalinternal reflection for directing said emitted light toward saiddichroic combiner; (c) at least one second wavelength LED mounted on asecond support element; (d) second collector optics for collectingemitted light from said at least one second wavelength LED and directingsaid emitted light from said at least one second wavelength LED towardsaid dichroic combiner and along said illumination axis; and (e) aspatial light modulator for modulating light received along saidillumination axis and for directing the light modulated thereby towardsa lens for focusing onto the photosensitive medium.
 8. A method forrecording images onto photosensitive medium, comprising: (a) emittinglight from at least one first wavelength LED, positioned on a firstsupport element; (b) collecting emitted light from said at least onefirst wavelength LED through first collector optics and directing saidemitted light from said at least one first wavelength LED through adichroic combiner and along an illumination axis, wherein the step ofdirecting said emitted light comprises the step of collimating emittedlight and using total internal reflection; (c) emitting light from atleast one second wavelength LED positioned on a second support element;(d) collecting emitted light from said at least one second wavelengthLED through second collector optics and directing said emitted lightfrom said at least one second wavelength LED toward said dichroiccombiner and along said illumination axis, wherein the step of directingsaid emitted light comprises the step of collimating emitted light andusing total internal reflection; and (e) modulating light from saidillumination axis at a spatial light modulator and directing the lightmodulated thereby towards a lens for focusing onto the photosensitivemedium.